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    SubjectRe: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section
    [+Tony]

    On Mon, Feb 11, 2019 at 02:34:31PM -0800, Linus Torvalds wrote:
    > On Mon, Feb 11, 2019 at 9:30 AM Will Deacon <will.deacon@arm.com> wrote:
    > >
    > > +
    > > + 1. All readX() and writeX() accesses to the same peripheral are ordered
    > > + with respect to each other. For example, this ensures that MMIO register
    > > + writes by the CPU to a particular device will arrive in program order.
    >
    > Hmm. I'd like more people look at strengthening this one wrt across
    > CPUs and locking.
    >
    > Right now we document mmiowb(), but that "documentation" is really
    > just a fairy tale. Very *very* few drivers actually do mmiowb() on
    > their own.
    >
    > IOW, we should seriously just consider making the rule be that locking
    > will order mmio too. Because that's practically the rule anyway.

    I would /love/ to get rid of mmiowb() because I think it's both extremely
    difficult to use and also pretty much never needed. It reminds me a lot of
    smp_read_barrier_depends(), which we finally pushed into READ_ONCE for
    Alpha.

    > Powerpc already does it. IO within a locked region will serialize with the
    > lock.

    I thought ia64 was the hold out here? Did they actually have machines that
    needed this in practice? If so, I think we can either:

    (a) Add an mmiowb() to their spin_unlock() code, or
    (b) Remove ia64 altogether if nobody complains

    I know that Peter has been in favour of (b) for a while...

    Will

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